1. Technical Field of the Invention
The present invention relates to a light beam scanning optical system, and more particularly to a structure of a light beam scanning optical system incorporated in laser beam printers, facsimiles and the like for scanning a beam receiving surface with a light beam.
2. Brief Description of Related Art
Generally, a light beam scanning optical system utilized in laser beam printers and facsimiles basically comprises a semiconductor laser as a light source, a deflecting means such as a polygon mirror and an f.theta. lens. The scanning means is utilized for scanning a luminous flux emitted from the semiconductor laser at an equiangular velocity, whereby a spot of the luminous flux moves on a light receiving surface in a main scanning direction. With the equiangular velocity, a difference in a scanning speed occurs over a range from central portion to both edges in the main scanning direction because the light receiving surface is flat. An f.theta. lens is therefore provided for rectifying the difference in the scanning speed.
Since the f.theta. lens must be made by combining various concave lenses and convex lenses, lens planning is extremely complicated. Because of a number of lens surfaces to be ground, manufacturing process of the f.theta. lens requires higher accuracy and higher cost. Moreover, the f.theta. lens has a problem that various lenses of the f.theta. lens must be made of a limited material which possesses good permeability. In order to solve the problems, in place of the f.theta. lens, the use of an elliptical mirror (Japanese Published Unexamined Patent Application No. 123040/1979), the use of parabolic mirror (Japanese Published Examined Patent Application No. 36127/1980) and the use of a concave reflector (Japanese Published Unexamined Patent Application No. 173212/1986) have heretofore been proposed. However, there still leaves difficulties in manufacturing the elliptical mirror and parabolic mirror with further difficulty in obtaining high manufacturing accuracy.